usb.c freezes the CPU when there is no one listening

[anacierdem]

Without the following timeout, the busy function is waiting indefinitely when the serial listener is hung up or not started at all.

static void usb_everdrive_usbbusy() 
{
    u32 timeout = 0;
    u32 val __attribute__((aligned(8)));
    do
    {
        usb_everdrive_readreg(ED_REG_USBCFG, &val);
        if (timeout++ != 8192)
            continue;
        usb_everdrive_writereg(ED_REG_USBCFG, ED_USBMODE_RDNOP);
    } while ((val & ED_USBSTAT_ACT) != 0);
}

I am not sure if it is possible to stop listening with the original UNF executable (I presume it is the -d flag?) but it should happen if you are logging something say every frame and you terminate the process on computer side. Previously the above fix was removed with #85 as it was causing issues for the original SDK. The fix still exists in libdragon.

An important difference between the two implementation is that, libdragon does not have threading yet so that might be causing something to break. Maybe having two different implementations for libdragon behind a define guard is an acceptable solution until the root cause is found?

[buu342]

Fixed in b716f17.